The present invention relates to electronically monitoring the mechanical oscillation characteristics of one or more resonantly vibrating tubes through which flowable substances are passing to determine the density of the substances. Both the temperature and natural period of oscillation of one or more resonantly oscillating tubes are electronically monitored, and the resulting signals are processed to accurately determine the density of the flowing substances independent of temperature variations.
It is known that one end of an elastic container can be rigidly mounted and that the other end of the cantilever mounted container can be oscillated at a resonant angular velocity. For such a spring-mass mechanical system, the resonant angular velocity of oscillation can be calculated by taking the square root of the ratio of the oscillating container's mass to its spring constant about the oscillation axis. Since the mass of the container will vary as the density of the substances flowing through the container varies, monitoring the resonant angular velocity of oscillation of a cantilever mounted container potentially offers a convenient means for measuring the density of flowable substances.
Various container configurations have been applied to the construction of oscillating container densimeters. Among these has been U-shaped flow tubes, with the tube rigidly mounted to a fixed base at the open ends of the U-shaped flow tube. Thus, by flowing a substance through the U-shaped flow tube and vibrating the tube at its resonant angular velocity the density of substances flowing through the U-shaped flow tube can be measured. A known improvement for the vibrating U-shaped flow tube densimeter is the use of a second cantilever mounted U-shaped flow tube which is positioned along side the first U-shaped flow tube so as to have both U-shaped flow tubes act as the times of a tuning fork. Among the advantages achieved by such tuning fork operation is substantial attenuation, at the base, of vibration forces associated with the driving of the U-shaped flow tubes at their resonant angular velocities of oscillation.
An additional variable which affects measurements of the density of flowable substances passing through oscillating container densimeters is the temperature of the oscillating container. For, the spring constants of an elastic container are dependent on temperature. This dependency on temperature and its effect on the determination of density by measuring resonant angular velocity of oscillation has been known. Previous attempts to address the dependency of the resonant angular velocity of oscillation on temperature variations have included those disclosed in U.S. Pat. Nos. 4,170,128 and 3,910,101. Disclosed in U.S. Pat. No. 4,170,128 is a mechanical arrangement incorporating a length of cord or string attached to a vibrating U-shaped flow tube. According to the disclosure in U.S. Pat. No. 4,170,128, the length of the string is dependent on temperature, and, therefore, the tension of the mounted string is varied by temperature. As further disclosed this variation in tension modifies the vibration characteristics of the U-shaped flow tube to compensate for temperature variations. Disclosed in U.S. Pat. No. 3,910,101 is an electrical circuit for both monitoring and controlling the oscillation of a fluid filled container. Included as a part of the disclosed circuit is a bridge circuit having as the variable element a temperature dependent sensor. The output of the bridge circuit according to the teachings in U.S. Pat. No. 3,910,101 is used to alter, according to fluctuations in temperature, both the phase and amplitude of the signal driving the oscillation of the fluid filled container.
The disclosures in both U.S. Pat. Nos. 4,132,110 and 3,910,101 are directed to the monitoring of temperature variations for the purpose of altering the mechanical oscillations of oscillating containers.
Unlike the previous two patents U.S. Pat. No. 4,132,110 discloses a mechanical configuration and electrical circuit for determining the density of a fluid passing through an oscillating container without a feedback system for altering the mechanical oscillations of the container as a function of temperature variations. The electrical circuitry disclosed in U.S. Pat. No. 4,132,110 for determining fluid density incorporates an oscillator to generate a standard signal from which is subtracted the signal representative of the motion of the oscillating container. According to the disclosures in U.S. Pat. No. 4,132,110, this difference signal is then combined with the signal from a temperature probe to provide a signal proportional to the density of the fluid being measured based on a reference temperature.